1. Field of the Invention
The present invention relates to display device fabrication, and more particularly to an apparatus and its fabrication methods which employ modular active devices which are separately fabricated from and assembled on a substrate.
2. Description of the Related Art
Flat panel displays, such as OLEDs (Organic Light Emitting Diodes) or AMLCDs (Active Matrix Liquid Crystal Displays), are currently manufactured in a process where the light emitting or transmitting elements are built simultaneously with active electrical devices and addressing electrodes. All these elements are formed in their final desired position on a common substrate. These active electrical devices are formed at each subpixel element of an array of subpixels. This approach has significant economic disadvantages in that the processing required for fabrication of the active electrical devices is much more complex than that required for fabrication of the pixel electrode for light emitting or transmitting elements and the addressing electrodes.
In one example, a passivated amorphous silicon thin film transistor (TFT) can be formed in four mask steps, and if the addressing lines do not include cross-overs and passivation is not required, then only two mask steps are required to pattern aluminum conductors and transparent Indium Tin Oxide (ITO) pixel electrodes for the light emitting or transmitting elements. In a typical direct view OLED or AMLCD display, the active electrical devices only occupy about 10% or less of the total substrate area.
Significant cost savings could be achieved if the active electrical devices could be fabricated separately from the substrate including the pixel electrodes and the addressing electrodes, and subsequently assembled onto the substrate including the pixel and the addressing electrodes if a low cost and compact joining technology was used.
One approach for assembling small pieces of silicon chips containing the active electrical devices onto a display substrate is fluidic self assembly described by Alien Technologies (See, http://www.alientechnology/com/library/pdf/fsa_white_paper.pdf). In this process, the display substrates are formed from plastic and include indentations where it is desirable to locate active electrical devices. The indentations match the shape of the small pieces of silicon chips. The small chips are suspended in a fluid which flows over the substrate and deposits the chips in the desired locations. Electrical connections are formed by depositing metal over the chips including the active electrical devices and the substrate and patterning the metal. This approach has significant disadvantages in that the indentations in the substrate and the small chips including the active electrical devices must be precisely shaped to match and there is no easy means of replacing the small chip including the active devices or reworking the electrical contact to the substrate if a small chip including the active devices is damaged during assembly and processing or an electrical contact fails.
Further, the precise shape of the small silicon chip including the active devices is achieved by photolithography in combination with anisotropic etching of single crystalline silicon, so expensive silicon wafers must be used for fabrication of the active devices.
Therefore, a need exists for an apparatus and method of fabrication which includes separately fabricated active devices integrated on a substrate for a display device for reducing costs and manufacturing complexity.